schema cache: Store and check info for pre-schema servers
    
    Cache CommandError answer to schema command to avoid sending the command
    to pre-schema servers every time. This information expires after some
    time (1 hour) in order to start using schema as soon as the server is
    upgraded.
    
    https://fedorahosted.org/freeipa/ticket/6095
    
    Signed-off-by: Jan Cholasta <jcholast@redhat.com>
    Signed-off-by: David Kupka <dkupka@redhat.com>
    Reviewed-By: Tomas Krizek <tkrizek@redhat.com>